Particle-filled microporous materials

ABSTRACT

A microporous particulate-filled thermoplastic polymeric article is provided. The article can be in the form of a film, a fiber, or a tube. The article has a thermoplastic polymeric structure having a plurality of interconnected passageways to provide a network of communicating pores. The microporous structure contains discrete submicron or low micron-sized particulate filler, the particulate filler being substantially non-agglomerated.

The Government of the United Sates of America has rights in thisinvention pursuant to Subcontract 6678305 under Contract W-7405-ENG-48awarded by the Department of Energy.

FIELD OF THE INVENTION

The present invention relates to particle-filled microporous materialssuch as films and fibers, a method for preparing same and articlesprepared therefrom.

BACKGROUND OF THE INVENTION

It has been common practice to use fillers in polymers to produce alarge variety of articles. Such articles contain a range of fillers suchas, for example, highly stable and color-fast pigments, activated carbonsorbents, ion exchange resins, and fine silver particles forphotographic films.

U.S. Pat. No. 2,947,646 (Devaney et al.) discloses colloidal dispersionsof metals in plastics which are prepared by depositing a thin coating ofmetal onto finely powdered plastic, working the metal-coated plasticpowder into a plastic state, the working resulting in fragmentation ofthe metal coating into very small metallic particles, and fabricatinginto the final shape.

U.S. Pat. No. 3,082,109 (Devaney et al.) discloses colloidal dispersionsof metals in plastic which are prepared by incorporating into plasticmetals which melt at or below the temperature of the metal rolls usedfor compounding the plastic material, compounding sufficiently todisperse the melted metals throughout the plastic, and fabricating intothe final shape.

British Pat. Specification No. 1,100,497 describes the production ofpolymer fibers from polymer solutions wherein a solution of the polymeris formed at an elevated temperature, below the decompositiontemperature of the polymer, in at least one non-polymeric compound whichdissolves the polymer at the elevated temperature and which does notdissolve the polymer at a lower temperature and the solution is extrudeddownwardly through a spinneret into unheated air which cools thesolution to the lower temperature causing separation of the polymer fromthe solvent to form a fiber. The fiber may contain fillers such assulfur, carbon black, or an ion exchange resin.

M. C. Williams and A. L. Fricke, "Phase Separation Spinning ofPolypropylene," SPE Journal, 28, Oct. 1972, p. 51, describe a method ofmaking porous fibers by spinning a hot solution of polypropylene innaphthalene, allowing the spun solution to cool to solidify the fiberand effect phase separation of polypropylene and naphthalene, andremoving the naphthalene by extraction with diethyl ether. Williams andFricke suggest that this technique could possibly be used to producefibers with high filler contents by mixing solid fillers into thespinning solution.

U.S. Pat. No. 3,351,495 (Larsen et al.) discloses a battery separatorcomprising a microporous sheet of a specified polyolefin. The batteryseparator, which preferably contains a homogeneous mixture of 8 to 100volume percent of polyolefin, 0 to 40 volume percent plasticizer, and 0to 92 volume percent inert filler material, is prepared by blending thecomponents and then fluxing the blend in a conventional mixer such as aBanbury mixer or melt homogenizing the blend in a conventional two rollmill, forming the composition into sheet form, and extracting at least aportion of the inert filler and/or plasticizer.

U.S. Pat. No. 4,650,730 (Lundquist et al.) describes a battery separatorhaving at least two plies, each in the form of a microporous sheet, atleast one ply, formed from a polymeric composition comprising a polymerand, optionally, plasticizers, stabilizers, antioxidants, and the likebut substantially no particulate filler and being capable oftransforming to a substantially non-porous membrane at a temperaturebetween about 80 and 150° C., and at least one ply, formed from apolymeric composition comprising a polymer and, optionally,plasticizers, stabilizers, antioxidants, and the like and preferablycontains a large amount (greater than 20) weight percentage of solid,particulate filler.

U.S. Pat. No. 3,745,142 (Mahlman et al.) describes a process forpreparing highly filled polyolefins which comprises preparing acrystallizable olefin polymer having a specified particle size, addingto the olefin polymer about 50 to 1900%, based on the weight of olefinpolymer of a solid, particulate, inorganic filler material which isinsoluble in the olefin, which is solid at the melting point of theolefin polymer and which is in the form of particles of about 0.1 to 25microns, shaping the resultant polymer-filler blend, and fusing theolefin polymer to from a continuous phase. The highly filledcompositions can be prepared simply by adding the particulate inorganicfiller directly into a dispersion, either aqueous or organic, of theolefin polymer and agitating to make the total dispersion uniform.

British Pat. Specification No. 1,327,602 (Hercules) describes a processfor preparing filled olefin polymers containing about 35 to 90% byweight of a particulate, inert, inorganic filler comprising extruding asubstantially homogeneous mixture of polymer, filler and 25 to 75% byweight of the homogeneous mixture of hydrocarbon wax, which acts as alow viscosity diluent for the polymer, to form a shaped structure,cooling the extruded blend to form a continuous polymer phase,extracting the wax, and recovering the filled structure in the shapedesired.

U.S. Pat. Nos. 4,342,811 and No. 4,550,123 (Lopatin et al.) describeopen-celled microporous sorbent-loaded textile fibers and films preparedby forming a melt blend, of the sorbent particles, the polymer and aselected diluent, e.g., in a batch-type blender such as a Sigma blademixer or blending extruder such as a twin-screw compounding extruder,spinning or extruding and drawing down the fiber or film, and extractingthe diluent.

U.S. Pat. No. 4,562,108 (Miyake et al.) describes a heat-retainingmoisture-transmissible water-resistant fabric having a fibroussubstrate, a discontinuous polymer layer or a polymer layer having amultiplicity of interconnecting fine pores, and a polymer layercontaining 15 to 70 weight percent, based on the weight of the polymer,of heat ray-reflecting fine metal pieces and having interconnecting finepores. Alternatively, a specified microporous film layer may beinterposed between the polymer layers. The metal-containing layer isprepared by solvent casting the polymer after simple mixing with analuminum paste.

In addition it has been known for a long time to use dyes and pigmentsin polymers for coloration. Products made using this art usecommercially available pigment dispersions to avoid pigmentagglomeration and the articles formed are usually of non- porouspolymers. See D. Bennett, Nonwoven World, 2, Nov. 1987, p. 58.

Metal or metal oxide filled polymeric fabrics for X-ray absorption havebeen widely used to construct personal protective garments for a numberof years. U.S. Pat. No. 3,514,607 describes composite shields againstlow energy X-rays which are sheets of a carrier material containing tin,antimony, iodine, barium, or a combination thereof and lead. The carriermaterial may be flexible, e.g., a plastic or rubber material, or rigid,e.g., a plastic or a building material. The minimum content of carriermaterial needed to yield materials with acceptable mechanical strengthis 16% by weight.

U.S. Pat. No. 4,619,963 and No. 4,485,838 (Shoji et al.) describe aradiation shielding composite sheet material of melt-spun lead fibers ofmore than 99% purity, and containing 50 to 500 ppm tin, of a mean lengthof 0.5 to 1.3 mm which are embedded in a synthetic resin, such that thecomposite sheet has a specific gravity greater than 4.0. The sheetmaterial can be formed by melt-spinning the tin-containing lead fibersat a diameter below 60 microns, cutting the fibers to a length of 0.5 to1.3 mm in length, blending the fibers with a thermoplastic resin, e.g.,in a Banbury mixer, and pressing the blend between rolls to form asheet. Efficacy data is given comparing these constructions to powderfilled composites made by the same process. The powder filled compositescan be made with up to 75 weight percent lead and can absorb X-rays upto 60 percent as well as crystalline lead foil. The fiber filledcomposites can be made with up to 85 weight percent lead and absorb with80 percent the efficiency of lead foil. Lead filler levels of 75 and 85weight percent are 23 and 35 volume percent respectively.

U.S. Pat. No. 4,247,498 (Castro) discloses microporous polymerscharacterized by a relatively homogeneous, three-dimensional cellularstructure having cells connected by pores of smaller dimension. Themicroporous polymers are prepared from thermoplastic polymers by heatinga mixture of the polymer and a compatible liquid to form a homogeneoussolution, cooling the solution under non-equilibrium thermodynamicconditions to initiate liquid-liquid phase separation, and continuingthe cooling until the mixture achieves substantial handling strength.

U.S. Pat. No. 4,539,256 (Shipman) discloses a microporous sheet materialcharacterized by a multiplicity of spaced randomly dispersed, equiaxed,non-uniform shaped particles of the thermoplastic polymer, adjacentthermoplastic particles connected to each other by a plurality offibrils of the thermoplastic polymer. The sheet materials are preparedby melt blending crystallizable thermoplastic polymer with a compoundwhich is miscible with the thermoplastic polymer at the meltingtemperature of the polymer but phase separates on cooling at or belowthe crystallization temperature of the polymer, forming a shaped articleof the melt blend, cooling the shaped article to a temperature at whichthe polymer crystallizes to cause phase separation to occur between thethermoplastic polymer and the compound.

U.S. Pat. No. 4,726,989 (Mrozinski) discloses microporous materialsincorporating a nucleating agent made by melt blending a crystallizablethermoplastic polymer with a nucleating agent which is capable ofinducing subsequent crystallization of the thermoplastic polymer andwith a compound which is miscible with the thermoplastic polymer at themelting temperature of the polymer but phase separates on cooling at orbelow the crystallization temperature of the polymer, forming a shapedarticle of the melt blend, cooling the shaped article to a temperatureat which the nucleating agent induces the thermoplastic polymer tocrystallize so as to cause phase separation to occur between thethermoplastic polymer and the compound.

SUMMARY OF THE INVENTION

The present invention, in one aspect provides a microporousparticulate-filled thermoplastic polymeric article which comprises athermoplastic polymeric structure having a plurality of interconnectedpassageways to provide a network of communicating pores, the microporousstructure containing discrete submicron or low micron-sized particulatefiller, the particulate filler being substantially non-agglomerated.

The thermoplastic polymeric structure may be substantially homogeneousthroughout or the porosity of the structure may be gradienttherethrough. The particulate filler may be substantially uniformlydistributed throughout the article or the particulate filler may have agradient density throughout the article.

The microporous particulate-filled articles may be provided as, forexample, films, fibers, hollow fibers, and tubes. When the structure isin the form of a film, the film may be uniaxially or biaxially oriented.When the structure is in the form of a fiber, hollow fiber or tube, itmay also be oriented. The articles of the invention have a network ofinterconnected passageways to provide communicating pores, with higheffective pore size range, low fluid flow resistance, broad pore sizecontrol and with up to 50 or more volume percent filler loading.

The present invention, in a further aspect, provides a microporousparticulate-filled thermoplastic polymeric filtration media comprising athermoplastic polymeric structure having a plurality of interconnectedpassageways to provide a network of communicating pores, the microporousstructure containing discrete submicron or low micron-sized particulatefiller, the particulate filler being substantially non-agglomerated.

The present invention, in another aspect, provides a protective garmentcomprising a microporous particulate-filled thermoplastic polymericfabric, the fabric comprising a thermoplastic polymeric structure havinga plurality of interconnected passageways to provide a network ofcommunicating pores, the microporous structure containing discretesubmicron or low micron-sized particulate filler, the particulate fillerbeing substantially non-agglomerated. The microporous particulate-filledthermoplastic polymeric material may be bonded or laminated to a film, awoven, knitted, or nonwoven fabric or scrim to provide additionalstructural stability and durability.

The present invention, in a further aspect, provides X-ray shieldingmaterial comprising a thermoplastic polymeric structure having aplurality of interconnected passageways to provide a network ofcommunicating pores, the microporous structure containing discretesubmicron or low micron-sized heavy metal X-ray absorbing particulatefiller, the particulate filler being substantially non-agglomerated.Surprisingly, even when the densities of the X-ray shielding material ofthe invention are less than 0.5 percent of those of the heavy metalshielding, the materials of the invention are as efficient as a foil ofthe crystalline metal on a comparable weight per area basis.

The present invention, in a further aspect, provides electromagneticshielding material comprising a thermoplastic polymeric structure havinga plurality of interconnected passageways to provide a network ofcommunicating pores, the microporous structure containing discretesubmicron or low micron-sized heavy metal electromagnetic absorbingparticulate filler, the particulate filler being substantiallynon-agglomerated. The particulate filler preferably absorbselectromagnetic waves having frequencies in the range of microwaves toX-rays.

The present invention, in a further aspect, relates to a method forpreparing a particulate-filled microporous thermoplastic polymericshaped article which comprises the steps of

(a) dispersing submicron or micron-sized particulate filler in a liquidcompatible with the thermoplastic polymer to form a colloidal suspensionof the particulate filler in the liquid, the particulate filler beingsubstantially non-agglomerated;

(b) melt-blending the thermoplastic polymer with a solubilizing amountof the compatible liquid containing the dispersed particulate filler ata temperature sufficient to form a homogeneous solution;

(c) forming an article from the solution;

(d) cooling the shaped article at a rate and to a temperature sufficientto initiate thermodynamic, non-equilibrium phase separation;

(e) further cooling the article to solidify the thermoplastic polymer;and

(f) removing at least a substantial portion of the compatible liquidwith the particulate filler remaining substantially entirely within thethermoplastic polymer article.

The article can optionally be oriented after solidification of thethermoplastic polymer, step e, or after removal of the compatibleliquid, step f. The process of the invention enables large volumes offiller to be effectively introduced into the article with relativelylarge volumes of the compatible liquid without loss of mechanicalproperties since the compatible liquid is not present in the finalarticle and the removal of the compatible liquid is accomplished withoutsubstantial removal of the filler from the article.

The thermodynamic, non-equilibrium phase separation may be eitherliquid-liquid phase separation or liquid-solid phase separation.

When liquid-liquid phase separation occurs, the cells comprise voidspaces encased by fibrous, lacy, or semi-continuous boundaries with thefiller particles attached thereto. Upon orientation, the cells becomeelongated in the direction of orientation. The cells of the orientedarticle are generally ellipsoidal in shape with an aspect ratio of majoraxis to minor axis greater than 1.0 and a major axis generally lying ina plane parallel to the surface of the article. The filled particlesreside in and are attached to the thermoplastic polymer of the formedstructure.

When liquid-solid phase separation occurs, the material has an internalstructure characterized by a multiplicity of spaced, randomly disposed,non-uniform shaped, equiaxed particles of thermoplastic polymer,adjacent particles throughout said material being separated from oneanother to provide the material with a network of interconnectedmicropores and being connected to each other by a plurality of fibrilsconsisting of the thermoplastic polymer. The fibrils elongate uponorientation providing greater spacing between the thermoplastic polymerparticles and increased porosity. Again, the filled particles reside inand are attached to the thermoplastic polymer of the formed structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a temperature-composition plot for the thermoplasticpolymer/compatible liquid systems of the invention.

FIG. 2 is a photomicrograph at 2000X of the prior art copper-filledmicroporous polyethylene film of Comparative Example 2.

FIG. 3 is a photomicrograph at 2000X of the tungsten-filled microporouspolyethylene film of Example 1.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term "thermoplastic polymer" refers only toconventional polymers, both crystalline and non-crystalline, which aremelt processable under ordinary melt processing conditions and does notinclude polymers such as polytetrafluoroethylene which, only underextreme conditions, may be thermoplastic and melt processable.

As used herein, the term "crystalline", as used with regard to thethermoplastic polymer, includes polymers which are at least partiallycrystalline. Crystalline polymer structures in melt-processedthermoplastic polymers are well known.

As used herein, the term "amorphous", as used with regard to thethermoplastic polymer, includes polymers without substantial crystallineordering such as, for example, polymethylmethacrylate, polysulfone, andatactic polystyrene.

As used herein, the term "melting temperature" refers to the temperatureat which the thermoplastic polymer, in a blend of thermoplastic polymerand compatible liquid, will melt.

As used herein, the term "crystallization temperature" refers to thetemperature at which the thermoplastic polymer, in a melt blend ofthermoplastic polymer and compatible liquid, will crystallize.

As used herein, the term "equilibrium melting point", as used withregard to the thermoplastic polymer, refers to the commonly acceptedmelting point temperature of the thermoplastic polymer as found inpublished literature.

As used herein, "particle" refers to submicron or low micron-sizedparticles, also termed "particulate filler" herein, such particleshaving a major axis no larger than five microns.

As used herein, "discretely dispersed" or "colloidal suspension" meansthat the particles are arrayed substantially as individual particleswith uniform spacings throughout a liquid or solid phase.

The submicron or low micron-sized particles useful in the presentinvention are capable of forming a colloidal dispersion with thecompatible liquid and insoluble in the melt blend of the thermoplasticpolymer and compatible liquid from which the articles of the inventionare formed. The submicron or low micron-sized particles are preferablyless than 5 microns in diameter, more preferably less than 3 microns indiameter, and most preferably less than about 1 micron in diameter.Useful particles include metals such as, for example, lead, platinum,tungsten, gold, bismuth, copper, and silver, metal oxides such as, forexample, lead oxide, iron oxide, chrome oxide, titania, silica andaluminia, and blends thereof carbonaceous materials such as, forexample, carbon black. Thermoplastic polymers useful in the presentinvention include olefinic, condensation and oxidation polymersRepresentative olefinic polymers include high and low densitypolyethylene, polypropylene, polyvinyl-containing polymers,butadiene-containing polymers, acrylate-containing polymers such aspolymethyl methacrylate, and fluorine containing polymers such aspolyvinylidene fluoride. Condensation polymers include polyesters suchas polyethylene terephthalate and polybutylene terephthalate, polyamidessuch as nylon 6, nylon 11, nylon 13, and nylon 66, polycarbonates andpolysulfones. Polyphenylene oxide is representative of the oxidationpolymers which can be used. Blends of thermoplastic polymers may also beused.

The compatible liquid is a material which is capable of forming asolution with the thermoplastic polymer when heated above the melttemperature of the polymer and which phase separates from the polymer oncooling. The compatibility of the liquid with the polymer can bedetermined by heating the polymer and the liquid to form a clearhomogeneous solution. If a solution of the polymer and the liquid cannotbe formed at any liquid concentration, then the liquid is inappropriatefor use with that polymer. In practice, the liquid used may includecompounds which are solid at room temperature but liquid at the melttemperature of the polymer. Generally, for non-polar polymers, non-polarorganic liquids with similar room temperature solubility parameters aregenerally useful at the solution temperatures. Similarly, polar organicliquids are generally useful with polar polymers. When blends ofpolymers are used, useful liquids are those which are compatible liquidsfor each of the polymers used. When the polymer is a block copolymersuch as styrene-butadiene, the liquid selected must be compatible witheach type of polymer block. Blends of two or more liquids can be used asthe compatible liquid as long as the selected polymer is soluble in theliquid blend at the polymer melt temperature and the solution formedphase separates on cooling.

Various types of organic compounds have been found useful as thecompatible liquid, including aliphatic and aromatic acids, aliphatic,aromatic and cyclic alcohols, aldehydes, primary and secondary amines,aromatic and ethoxylated amines, diamines, amides, esters and diesters,ethers, ketones and various hydrocarbons and heterocyclics. When thepolymer selected is polypropylene, aliphatic hydrocarbons such asmineral oil, esters such as dibutyl phthalate and ethers such asdibenzyl ether are useful as the compatible liquid. When high densitypolyethylene is the polymer, an aliphatic hydrocarbon such as mineraloil or and aliphatic ketone such as methyl nonyl ketone or an ester suchas dioctyl phthalate are useful as the compatible liquid. Compatibleliquids for use with low density polyethylene include aliphatic acidssuch as decanoic acid and oleic acid or primary alcohols such as decylalcohol. When the polymer is polyvinylidene fluoride, esters such asdibutyl phthalate are useful as the compatible liquid. When the polymerselected is nylon 11, esters such as propylene carbonate, ethylenecarbonate, or tetramethylene sulfone are useful as the compatibleliquid. When the polymer selected is polymethylmethacrylate, usefulcompatible liquids include, 1,4-butanediol and lauric acid. A compatibleliquid for use with the polymer polyphenylene oxide is, for example,tallowamine.

The amount of filler particles in the thermoplastic polymer depends uponthe amount of filler in the compatible liquid prior to melt blending andon the relative amount of thermoplastic polymer and compatible liquid inthe blend. The amount does not depend upon liquid removal because theparticles remain substantially entirely within the polymer structure.The amount of particles colloidally dispersed in the compatible liquiddepends upon how well the particles are wet by the liquid, the surfacearea of the particles, and the proper choice of a dispersing aid orsurfactant. Generally, for non-porous particle, a dispersion containing40-50 volume percent particles can be achieved. The amount of filler inthe polymer can be much greater than the amount of filler in thecompatible liquid when the melt blend has a higher concentration ofliquid than polymer.

The actual polymer concentration selected from within the predeterminedconcentration range for the liquid-polymer system being used is limitedby functional considerations. The polymer concentration and molecularweight should be sufficient to provide the microporous structure whichis formed on cooling with adequate strength for handling in furtherprocessing steps. The polymer concentration should be such that theviscosity of the liquid-polymer melt solution is suitable for theequipment used to shape the article. Generally, the polymerconcentration in the compatible liquid is about 10 to 80 weight percent,which corresponds to a compatible liquid concentration of 20 to 90weight percent. When high compatible liquid concentrations, i.e. 80 to90 percent, are used in conjunction with high volume percent of fillerin the compatible liquid, a very high, e.g., about 95 weight percent,concentration of the particulate filler in the thermoplastic polymer,relative to the liquid, can be achieved. For example, if the blend is90:10 liquid/polymer by volume and the liquid is 40 percent particulatefiller by volume, then the resulting filled microporous article is,surprisingly, 80 percent particulate filler by volume after the liquidis removed. That the particle-filled microporous thermoplastic polymericarticles of the invention can contain such large amounts of particulatefiller is unexpected because it is believed that particle-filledthermoplastic articles made by standard extrusion processes achieve onlyabout 20 percent filler by volume.

The relative amounts of thermoplastic polymer and compatible liquid varywith each system. The polymer concentration which can be used in a givensystem can be determined by reference to the temperature-compositiongraph for a polymer-liquid system as set forth in FIG. 1. Such graphscan be readily developed by known techniques such as set forth inSmolders, van Aartsen and Steenbergen, Kolloid-Z.u.Z. Polymere,243,14-20 (1971).

The portion of the curve from gamma to alpha represents thethermodynamic equilibrium liquid-liquid phase separation. T_(ucst)represents the upper critical solution temperature, i.e., the maximumtemperature of the system at which liquid-liquid phase separation willoccur. Φ_(ucst) represents the critical composition. To form themicroporous polymers of the present invention, the polymer concentrationutilized for a particular system must be greater than Φ_(ucst). If thepolymer concentration is less than this, the phase separation whichoccurs as the system is cooled forms a continuous liquid phase with adiscontinuous polymer phase, resulting in a structure which lackssufficient integrity.

The portion of the curve from alpha to beta represents equilibriumliquid-solid phase separation. Alternatively, the compatible liquid canbe chosen such that the thermoplastic polymer and compatible liquidsystem will exhibit liquid-solid phase separation or liquid-liquid phaseseparation over the entire composition range. For a given cooling ratein a system, the crystallization temperature-concentration curve of thecompatible liquid can be determined and from this curve theconcentration ranges for the polymer and the liquid which will yield thedesired microporous structure at the given cooling rate can bedetermined. The determination of the crystallization curve is analternative to determining the temperature-concentration phase diagramfor a system incorporating a semicrystalline polymer.

In the process of the present invention, the rate of cooling of thesolution may be varied within wide limits as long as the rate issufficient that the phase separation does not occur under thermodynamicequilibrium conditions. For many liquid-polymer systems, when the rateof cooling of the liquid-polymer solution is slow, but sufficient toresult in liquid-liquid phase separation, liquid-liquid phase separationoccurs at substantially the same time as the formation of a plurality ofliquid droplets of substantially uniform size. When the cooling rate issuch that the droplets form, the resultant microporous polymer will havea cellular microstructure. If the rate of cooling of the liquid-polymersolution is rapid, the solution undergoes a spontaneous transformationcalled spinodal decomposition, the resultant microporous polymer willhave a fine open-cellular microstructure. The fine microporous structureis referred to as a lacy structure. For many polymer systems whichinclude a crystalline polymer, when the rate of cooling is sufficient toresult in liquid-solid phase separation, the resulting microporouspolymer will have spherulitic microstructure. Thus, differingmicroporous structures can be obtained by either liquid-liquid orliquid-solid phase separation techniques by varying the cooling rate andthe liquid-polymer system used.

In the microporous structures of the invention, the particulate filleris uniformly and discretely arrayed in the structure. For example, whenthe structure is spherulitic, particles are in both the spherulites andin the fibrils between them. Although the particles are firmly held inthe polymeric structure, they are substantially exposed after liquidremoval. In a structure, the distribution of particles is uniformwherever the polymer phase occurs. The particles substantially exist asindividual, and not agglomerated, particles throughout the microporousstructure. Agglomerates of 3 to 4 particles may occur, but theirfrequency is no more than in the compatible liquid dispersion prior tomelt blending with the polymer. The average particle spacing dependsupon the volume loading of the particle in the polymer.

The compatible liquid is removed from the material to yield aparticle-filled, substantially liquid-free, microporous thermoplasticpolymeric material. The compatible liquid may be removed by, forexample, solvent extraction, volatilization, or any other convenientmethod, and the particle phase remains entrapped to a level of at leastabout 90 percent, more preferably 95 percent, most preferably 99percent, in the microporous polymer structure.

The particle-filled microporous structures of this invention can beoriented, i.e., stretched beyond their elastic limit to introducepermanent set or elongation and to ensure that the micropores arepermanently developed or formed. Orientation can be carried out eitherbefore or after removal of the compatible liquid. This orientation ofthe structures aids in controlling pore size and enhances the porosityand the mechanical properties of the material without changing theparticle uniformity and degree of agglomeration in the polymer phase.Orientation causes the microporous structure to expand such that theporosity increases.

Orientation of films of the invention can be used as a process variableto control thickness and relatively thin microporous films can beproduced. Thickness is particularly important for microporous filmapplications where selective fluid transport through the microporousfilm is desired, since the rate of transport is inversely proportionalto the thickness. Decreasing thickness minimizes the hydrostaticresistance to flow through the film. Orientation enables production ofthin films with minimal difficulty. Orientation also enhances themechanical strength of the films which is beneficial in many microporousfilm applications. With increasing orientation, film thickness andresistance to flow are proportionately reduced, mechanical strength andporosity are proportionately increased, and the pore size range isextended with improved pore size control, so that an excellent balanceof desired properties can be attained through selection of the amount oforientation to which the microporous film is subjected.

Particle-filled microporous films of the invention may be uniaxially orbiaxially oriented. Preferably, the particle-filled microporous filmsare stretched at least about 10 percent, more preferably about 10 to1000 percent. The actual amount of stretching required is dependent uponthe particular composition of the article and the degree of porositydesired. Stretching of the structure is preferably uniform so that theoriented structure has uniform, controlled porosity. When the structuresare uniaxially oriented, narrowing of the structure in the non-orienteddirection generally occurs, such that stretching the film for example, afilm, 50 percent does not result in a 50 percent increase in surfacearea, but something less than a 50 percent increase. Particle-filledmicroporous tubular film can be oriented, for example, usingconventional film stretching devices as well as by inflating the tubularfilm during the extrusion process thereby reducing film thickness andexpanding film dimensions radially. Particle-filled microporous fiberscan be oriented, for example, by stretching the fibers in a lengthwisedirection between a set of rolls with increasing speeds from 10 to 1000percent before or after removal of the compatible liquid. Theorientation is preferably dimensionally stabilized in the material usingwell-known techniques such as, for example, heating the material to astabilizing temperature under restraint. The presence of fillerparticles has little measurable effect on the orientation process or onthe mechanical properties of the resulting oriented article.

After removal of the compatible liquid and, optionally, orientation, theresulting particle-filled microporous material may be modified byimbibition of various materials, such as, for example, liquids, solventsolutions, solvent dispersions, or solids. Such materials may be imbibedby any of a number of known methods which result in the deposition ofsuch materials within the porous structure of the microporous material.The imbibed material may be physically entrapped within the microporousstructure, chemically reacted with the polymeric material of themicroporous structure, or attached to the particulate filler containedwithin the microporous structure. Examples of imbibing materials includemedicaments, fragrances, antistatic agents, surfactants, and pesticides.The thermoplastic polymer may be imbibed with a urethane monomer whichis then polymerized in place to provide a liquid-impermeable,vapor-permeable material.

The particle-filled microporous material of the invention may be furthermodified, either before or after removal of the compatible liquid, bydepositing various materials on the surface thereof using known coatingor deposition techniques. For example, the particle-filled microporousmaterial may be coated with metal by vapor deposition or sputteringtechniques or by materials such as adhesives, aqueous or solvent-basedcompositions, and dyes. Coating can be accomplished by such conventionalcoating techniques as, for example, roller coating, spray coating, dipcoating, and the like.

Particle-filled microporous sheet materials of the invention may belaminated to various other materials such as, for example, woven,knitted, or non-woven fabrics, films, or to one or more additionallayers of similar or other microporous sheet material to achieve, forexample, desired thicknesses, porosity gradients, handling properties,and aesthetics. Lamination can be carried out using conventionaltechniques such as adhesive bonding, spot welding, or other techniqueswhich do not undesirably interfere with the porosity or createundesirable porosity of perforations.

When particle-filled porous films of the present invention are preparedusing a polyolefin as the thermoplastic polymer, a preferred method ofproducing multi-layered laminates results in a composite film that hashigh surface to surface bond strength of the laminates, and nointerruptions of the porosity at the layer interfaces. The process usedto form these multi-layered filled microporous polyolefin sheets takesplace before stretching the polyolefin sheets and includes solventexchanging the aforementioned compatible liquid used for the particlefilled microporous film formation, for a volatile solvent, then placingthese wet sheets of polyolefin in intimate contact with each other,drying to remove the solvent and then stretching in one or moredirections. Alternatively, the particle filled polyolefin sheets withthe compatible liquid still in place, may be laid-up and placed inintimate contact with each other, the liquid removed by solventextraction with drying of the solvent, and then stretched in one or moredirections.

The particle-filled microporous material of the invention may bemodified to produce a porous membrane having a gradient porositytherethrough, if the extruded film is rapidly cooled from one surfacethereof immediately after extrusion, such as by bringing the surfaceinto contact with a chilled casting wheel. The surface of the film incontact with, for example, the chilled casting wheel can be fused orsealed, while the opposite side remains porous. Orientation of thisgradient porosity structure enhances the porosity differences fromsurface to surface. Films with such properties can be used, for example,for microfiltration or ultrafiltration or as protective films or tapes,having, for example, the porous side readily markable and the sealedside resistant to marking.

The particle-filled microporous materials of the invention are useful ina variety of applications where microporosity is desirable. For example,the microporous sheet materials can be used for ultrafiltration ofcolloidal matter, as filtering material for cleaning antibiotics, beer,oils, and bacteriological broths, and as diffusion barriers orseparators in electrochemical cells. The microporous sheet material canalso be used for sample collection in air analysis and for collection ofmicrobiological specimens. When laminated to woven scrim material, themicroporous materials can be useful for outerwear and for disposableprotective garments for use, for example, in nuclear power plants, inhospitals, electronic clean rooms, or in areas where contact withhazardous chemicals or radiation can occur. The microporous sheetmaterials are also useful in surgical dressings, bandages, and othermedical applications. In each of these applications the presence of afiller can enhance the performance of the material by adding eitherenergy absorption or chemical reactivity.

The selection of the particulate filler is determined by the specificapplication for which the particle-filled microporous membrane isintended. The particles must be submicron or low-micron in size and arepreferably chemically inert to the polymer and the compatible liquid.For example, tungsten and lead oxide are efficient absorbers ofhazardous radiation; aluminum and ferrite particles are useful forabsorbing microwave energy and converting the microwave energy to heat;certain transition metal oxides, such as chrome oxide, are useful forconverting solar energy to heat; and fine silver particles are useful asantibacterial agents.

The particle-filled microporous materials of this invention are usefulas an extremely low-density shielding material for X-rays when theparticulate filler is heavy metal-containing X-ray absorbingparticulate. Even when the densities of the X-ray absorbentparticulate-filled microporous porous films of this invention are lessthan 0.5 percent of those of the heavy metal shielding, the materials ofthe invention are as efficient as a foil of the crystalline metal on acomparable weight per area basis. Such low density composites of heavymetal X-ray absorbers is very desirable for production of practicalprotective fabric or garments.

The X-ray absorption efficiency of the materials of this inventionappear to be as high as that of the pure metal at the same basis weight.This 100 percent equivalency holds for extremely thin layers, about 0.01mm or less. This means that composites with the same absorption but lessweight than currently available can be made. The metal-filledmicroporous membranes are vapor-permeable which makes garments much morecomfortable for the wearer. The metal-filled porous membranes are soft,have a good hand, and are easily converted into garments even when themetal particle are present in high concentrations, e.g., 95 weightpercent.

This surprising and unexpectedly high X-ray absorption efficiency of theparticle-filled microporous articles of this invention is believed to bedue to the discretely dispersed phase of colloidal-sized particles whichare uniformly and discretely arrayed about the thermoplastic polymerphase. This degree of dispersion in the final article is a result ofachieving a colloidal dispersion of the filler particles in thecompatible liquid prior to melt-blending with the thermoplastic polymer.

In the first step of the process, the particulate filler, in powderform, is disposed beneath the surface of the compatible liquid and theentrained air is removed from the mixture. If this step is notspontaneous, a standard high speed shear mixer, such as made by SharInc., Fort Wayne, Ind., operating at several thousand RPM for about 60minutes can be used to achieve this step.

The second, more difficult and more important, step of the process ofthis invention, is breaking down agglomerated particles to their primaryparticle size within the compatible liquid. This second step can beaccomplished by milling, or grinding, the compatible liquid containingthe particulate material. Two types of mills useful for this millingare, for example, attritors and sand mills.

A surfactant is preferably added to the mixture of compatible liquid andparticulate filler to aid in dispersing the particulate filler in thecompatible liquid and in maintaining the particulate filler as discreteparticles. Anionic, cationic or nonionic surfactants can be used.Preferably, the surfactant is a low molecular weight polymer whichstabilizes the dispersion by steric interaction between the particles ora small ionic molecule which stabilizes the dispersion by a chargemechanism. Preferably, the surfactant is present in an amount of about 1to 40, more preferably 2 to 20, weight percent based on the weight ofthe particulate filler. Useful surfactants include OLOA 1200, apolyisobutene succinimide, available from Chevron Chemical Co., Houston,Tex., Wayfos™ TD-100, available from Phillip A. Hunt Chemical Co., EastProvidence, R.I., and Kr-55, available from Kenrich Petrochemicals,Bayonne, N.J.

Milling reduces agglomerates to primary particles but does not breakdown large particles to smaller particles. Therefore, filtration of themilled dispersion may be necessary, if large particles are present.

When a liquid containing particles is pulled or pushed through a filter,a complex flow system is set up in which little streams of liquid movefrom the bulk phase into and through the pores of the filter and out theother side. Liquid flows more readily through the larger pores, so thatthese pores are more likely to participate in the filtration process.Particles suspended in the liquid are carried along in the flowingliquid by inertia. If the particles are small enough to pass through theholes of the filter, they pass out the other side and become part of thefiltrate. Otherwise, the particles become impinged upon the surface ofthe filter, or become trapped within the interstices of the filtermatrix.

The following examples further illustrate this invention, but theparticular materials and amounts thereof in these examples, as well asother conditions and details, should not be construed to unduly limitthis invention. In the examples, all parts and percentages are by weightunless otherwise specified. Where stretch ratios are recited forextruded films, the first number indicates the stretch ratio in themachine direction (MD), i.e.,extrusion direction, and the second numberindicates the stretch ratio in the direction transverse to the extrusiondirection (TD).

In all of the examples to follow, the particle/compatible liquid millingwas carried out at a relatively high viscosity where the grinding, ormilling, process is much more effective. A typical dispersion containedabout 20 percent by volume of particles in compatible liquid, andtypical viscosity values were 10 poise at 0.1 sec⁻¹ shear rate and 5poise at 10 sec⁻¹ shear rate, as measured using a Rheometrics FluidRheometer Model 7800 under steady shear conditions. The grinding wascarried out using an attritor or the sand mill.

For making small batch volume dispersions, an attritor Model 6TSG-1-4,manufactured by Igarashi Kikai Seizo Co. Ltd., Tokyo, Japan, was used.This attritor is a water-cooled vessel about 1 liter in volume whichoperates at about 1500 RPM. The capacity of such an attritor is about500 cc of compatible liquid containing the particulate filler with agrinding media of about 300 cc of 1.3 mm diameter stainless steel balls.

For larger batches, a 0.5 gallon vertical sand mill manufactured bySchold Machine Co., St. Petersburg, Fla., was used. Typically, the sandmill would contain about 1300 cc of 1.3 mm stainless steel balls asgrinding media and operate at about 3000 RPM. In the sand mill, thedispersion being made is continuously fed from the bottom and exited outthe top by means of a gear pump. A typical recirculating rate for the0.5 gallon sand mill is 3 gallons per hour.

Generally, in the attritor and in the sand mill, grinding times of 4 to8 hours were required to achieve an adequate degree of dispersion,although, depending on the particulate used, grinding times of one houror less are sufficient. The degree of dispersion was monitored bysmearing a drop of the dispersion on a glass slide and viewing it with alaboratory microscope in a transmission mode at about 500X. At thismagnification, micron particles can be resolved visually. The dispersionwas deemed adequate when 95 percent or more of the particles in thedispersion existed as primary particles, rather than agglomerates.

A surfactant was utilized during the dispersion step to reduce viscosityat the higher volume loadings and to stabilize the dispersion towardflocculation. The surfactant utilized in most of the examples thatfollow was OLOA 1200, a polyisobutene succinimide, available fromChevron Chemical Co., Houston, Tex. Surfactant levels were about 10percent by volume of the particles. In addition to being an effectivedispersing aid for a variety of particle/organic liquid combinations, itis thermally stable at the melt blend temperatures utilized herein.

In the examples that follow, it was necessary to filter the dispersionsof particles in the compatible liquid to remove any large particles. AModel C3B4U 3 micron rope-wound filter made by Brunswick Technetics inTimonium, Md., was used just prior to melt-blending the dispersion withthe thermoplastic polymer. The filtration step also removed hardagglomerates that had not been reduced by the milling to diameters lessthan 3 microns. This resulted in a more uniform finished article andallowed the dispersions to be metered under pressure by close tolerancegear pumps during the extrusion process without frequent breakdowns dueto large particles clogging the pump. In all cases, less than 10 percentby weight of the dispersed particles was removed by this filtrationstep. After filtering, the concentration of particles in the compatibleliquid was determined by measuring density with a Model DMA-4SMettler/Paar density meter manufactured by Mettler Instrument Co.Hightsdown, N.J.

The following test methods were used in evaluating the various films:

Porosity (%)

The porosity is calculated according to the following formula: ##EQU1##where the bulk density is determined from measurement of specificgravity according to ASTM D-792.

Tensile Properties

Tensile strength (psi) and elongation (%) were measured according toASTM D-882 using an Instron Model 1122, available from Instron Corp.,Canton, Mass. under the following conditions:

jaw gap: 5 cm

crosshead speed: 50 cm/min

sample size: 2.5 cm wide

Filler concentration

Two different techniques were used in determining filler concentrationin the examples.

Inductively Coupled Plasma Spectroscopy (ICP)

With this technique, the sample was solubilized in a suitable solvent ata known concentration. The sample solution was then aspirated into aModel 3580 inductively coupled plasma torch, available from Bausch andLomb ARL, operating at 8000 to 10,000° C., which causes the atoms of thesample material to emit visible and ultraviolet radiation at wavelengthscharacteristic of the elements involved. The light intensity at eachcharacteristic wavelength is directly proportional to the concentrationof the source element in the sample solution being aspirated.

Differential Scanning Calorimetry (DSC)

In this technique, three scans (heat, cool, heat) are run on thematerial of interest and a control material produced under similarconditions but without the filler particles. The heat of fusion isdetermined for each of the scans and filler concentration calculatedusing the following equation: ##EQU2## where: X.sub.(filler) =wt-%filler in the sample ΔH_(f) (control) =heat of fusion of the controlsample (cal/g)

ΔH_(f) (filled)=heat of fusion of the filled sample (cal/g).

An average of the three scans is reported as the

filler concentration.

Structural Examination

The microstructure of the particle filled microporous materials wasexamined using an ISI model Super-lIIA scanning electron microscope(SEM). The materials were prepared for SEM analysis by freeze fracturingunder liquid nitrogen, mounting on SEM stubs, and vapor coating withapproximately 150 to 200 angstroms of pure gold. The size and spacialdistribution of the metal and metal oxide particles were determined byusing back-scattered electron imaging with the SEM on uncoatedmaterials. The resulting image is bright spots (resulting from the metaland/or metal oxide particles) on a dark background.

EXAMPLE 1

A mixture of 499 g of submicron tungsten powder, having an averageparticle size of about 0.5 micron, obtained from Union Carbide Corp.,Danbury, CT, 402 g white mineral oil having a density of 0.87 g/cc, and20 g Wayfos™ TD-100 surfactant, available from Phillip A. Hunt ChemicalCorp., East Providence, R.I., was prepared. A dispersion of the tungstenpowder in the mineral oil was achieved by milling with the Igarashiattritor for 6 hours, at 2000 RPM. Light microscopy at 300X showed adispersion comprised of primary particles having a diameter of less thanabout 1 micron. This dispersion was diluted with additional mineral oilto a concentration of 5 weight percent tungsten and filtered using a 3micron filter. Density measurements before and after filtering showed nosignificant loss of tungsten. The density of the dispersion was 0.93g/cc.

High density polyethylene, HDPE, from American Hoechst, product numberGM 9255, and the tungsten in mineral oil dispersion, were melt blendedto form a homogeneous mixture at the ratio of HDPE to dispersion of14.9:85.1 by volume. The blend was mixed and extruded using a 40 mmtwin-screw extruder at an extrusion rate of 18 kg/hr, and extruder screwspeed of 100 rpm, and a melt temperature of 160° C., through a film diehaving a slot 0.05 cm wide and 30.5 cm long. The extruded blend wascooled, by contacting a thermostated wheel maintained at 29° C. androtating at 3 m/min to initiate thermodynamic, non-equilibriumliquid-solid phase separation and solidification of the film. Thethickness of the film was 500 um.

The film was restrained in a frame and washed with 1,1,1-trichloroethaneto remove the mineral oil. The effluent was clear indicating thatsubstantially all of the tungsten particles remained with thepolyethylene. The restrained, washed film was dried to remove anyresidual 1,1,1-trichloroethane. The dried film was oriented bystretching at a temperature of 93° C., a preheat time of about 1 minute,a stretch rate of 30 cm/min, and a stretch ratio of 3:1 in the machinedirection and 3:1 in the transverse direction. While maintained at thisstretch ratio, the oriented film was then heat set at a temperature of100° C. The resulting tungsten-filled microporous polyethylene film wasevaluated for tungsten concentration using a differential scanningcalorimeter and for thickness and porosity. The results are set forth inTable 1.

                  TABLE 1                                                         ______________________________________                                        Thickness (microns):                                                                             103                                                        Porosity (%):      95.3                                                       Tungsten content (%):                                                                            17.2                                                       ______________________________________                                    

Scanning Electron Microscopy, SEM, analysis at a magnification of 5000Xof the porous filled polyethylene film showed a spherulitic structureindicative of liquid-solid phase separation, with spherulite diametersof about 2 microns spaced 1 to 5 microns apart. Back scattered SEManalysis at a magnification of 2000X showed that the tungsten particleswere discretely dispersed in the polymer, with most particles smallerthan 1 micron and none larger than 3 microns.

X-ray absorption efficiencies were measured using a Model Baltograph IV80 Kev Bremstrahlung source from Balteau Electrical Corporation,Stamford Conn. The efficiency of the tungsten filled porous films wascompared to a 600 nanometer thick foil of crystalline tungsten made bysputter deposition onto polyimide film. An equal tungsten basis weightof the tungsten filled porous film was achieved by stacking 8 layers ofthe prepared film of this example. The X-ray absorption efficiencies aregiven in Table 2.

                  TABLE 2                                                         ______________________________________                                                                      Absorption                                                     Tungsten Basis Weight                                                                        Efficiency                                      Sample         (mg/cc)        (%)                                             ______________________________________                                        600 nm W foil  1.16           17                                              8 layers W porous film                                                                       1.11           22                                              ______________________________________                                    

Control experiments showed that the X-ray absorption contributions fromthe polyimide film in the foil sample and from the polyethylene in the 8layers of porous film, were less than one percent. These results showthat a small amount of sub-micron tungsten particles, dispersedthroughout a volume 2000 times larger than an equivalent volume ofcrystalline tungsten foil, has an absorption efficiency equivalent tothat of the foil. This, in turn, shows that the tungsten-filledmicroporous film of this example has an extremely uniform dispersedphase of sub-micron particles on a scale of the primary particle size,i.e., 1.0 micron.

EXAMPLE 2

Tungsten powder, 41.65 Kg, 0.8 to 1.0 micron, available from TeledyneWah Chang, Huntsville, Ala., was mixed with 785 g OLOA surfactant, and6.63 Kg mineral oil, and was milled for 10 hours at 3000 RPM using the0.5 gallon vertical sand mill and filtered. The final density of themixture was 4.02 g/cc. Porous high density polyethylene film wasprepared as described in Example 1 using a melt blend volume ratio of13.9:86.1 polymer to dispersion. The cast film was washed with1,1,1-trichloroethane to remove the mineral oil, dried, and thenbiaxially oriented as described in Example 1. The film was characterizedfor thickness, porosity, and metal content, and the data is shown inTable 3.

                  TABLE 3                                                         ______________________________________                                        Thickness (microns):                                                                             210                                                        Porosity (%):      92                                                         Metal content (%): 95                                                         ______________________________________                                    

Various multilayer laminates of films of this example were then made.Prior to orientation, the sheets of the cast film were solvent exchangedsuch that the mineral oil was replaced with 1,1,1-trichloroethane. Thesolvent exchanged sheets were then placed in intimate contact with eachother, and dried to remove the 1,1,1-trichloroethane while beingrestrained rigidly at the periphery of the laminate. The laminates werethen biaxially oriented as described in Example 1. Attempts to separatethe layers of the porous material manually before or after biaxialorientation indicated the layers were bonded well and could not beseparated. Porosity measurements indicated no interruptions of theporosity at the layer interfaces.

The 80 Kev Bremstrahlung source described in Example 1 was used tomeasure the X-ray absorption efficiency of a 6 layer laminate having atungsten basis weight of 0.120 g/cm² : The measured absorptionefficiency was 99 percent. The 6-layer laminate had the same drape andhand as a single layer and, in fact, did not seem to be a laminate atall. This shows that a laminate, made without adhesive bonding, is auseful, lightweight, breathable fabric that is an efficient X-rayabsorber.

Additional X-ray absorption characterization was done with an I-129radio-isotope source. This source is monochromatic at 30 Kev. Theabsorption efficiency for a single layer of the film of this examplewith a basis weight of 20 mg/cm², was normalized to the number of layers(or basis weight) required to absorb 1 HVL (half value layer). 1 HVL isthe amount of a given material required for 50 percent absorptionefficiency. The measurement showed that the biaxially oriented, tungstenporous film of this example has a 1 HVL of I-129 absorption at athickness of 1.5 laminates or 30 mg/cm². The Radiological HealthHandbook, U S. Department of Health Education and Welfare, Rockville,Md., January 1970, gives I-129 HVL values for various metals and thevalue given for tungsten is 30 mg/cm, This basis weight of tungsten isequivalent to 0.015 mm of crystalline tungsten, therefore, the datashows that the filled porous film of this invention is as an efficientabsorber as crystalline tungsten at extremely low thicknesses. For manyapplications involving radioactive materials in the workplace,protective garments supplying 5 HVL of absorption are necessary. Thisdata shows that an 8 layer laminate of the material of this example willprovide this, and thus is a practical radiation protection fabric.

EXAMPLE 3

Lead oxide (PbO) powder (litharge) with an average diameter of 1.5microns was obtained from Hammond Lead Products, Hammond, Ind. Adispersion with a density of 2.42 g/cc was prepared by milling in thevertical sand mill for 8 hours, a mixture of 20.6 Kg PbO, 790 g OLOA,and 6.6 Kg mineral oil at 3000 RPM. A PbO-filled porous high densitypolyethylene film was prepared as in Example 2. Characterization data isgiven in Table 4.

                  TABLE 4                                                         ______________________________________                                        Thickness (microns):                                                                             175                                                        Porosity (%):      94                                                         PbO content (%):   93                                                         ______________________________________                                    

A PbO content of 93 weight percent is a filler content of 57 volumepercent, a remarkably high loading. The limiting factor on filler levelsin many processes is the loss of mechanical properties at about the 20volume percent level. This is is believed to be due to the fillerparticles not being sufficiently dispersed in the polymer. Although gooddispersion in plasticizers are easily achieved, high plasticizercontents also tend to deteriorate mechanical properties. The strength ofthe process of this invention is that large volumes of filler can beintroduced effectively with relatively large volumes of plasticizer,i.e., the compatible liquid, without loss of mechanical propertiesbecause the plasticizer is not present in the final article. Further,this removal is accomplished without substantial removal of the fillerfrom the porous thermoplastic matrix.

Multilayer laminates of the biaxially oriented film were made and testedfor X-ray absorption as described in Example 2. Using the 80 KevBremstrahlung source, a 9-layer sample containing 70 mg/cm² PbO showedan absorption efficiency of 97 percent. Using the 30 Kev monochromaticI-129 source, 1 HVL absorption was measured for a 4-layer laminatecontaining 31 mg/cm² of PbO.

EXAMPLE 4 AND COMPARATIVE EXAMPLE C1

In Example 4, a dispersion of tungsten particles, 0.8 to 1.0 microndiameter of Example 3, was prepared using 7.25 Kg of tungsten, 254 gOLOA surfactant, and 4.79 Kg mineral oil using the 0.5 gallon verticalsand mill operating at 3000 RPM for 7 hours. This mixture was dilutedwith mineral oil to a final density of 1.75 g/cc. Filled porous highdensity polyethylene film was extruded and cast as described in Example1 using a melt blend volume ratio of 33:67 polymer to dispersion. Aftercasting and cooling to effect liquid-solid phase separation and filmsolidification, the film was fed continuously into an extractor tankcontaining 1,1,1-trichloroethane to remove the mineral oil. Theextraction tank was plumbed such that fresh 1,1,1-trichloroethane wascontinuously added and the 1,1,1-trichloroethane which was saturatedwith mineral oil was removed counter-current to the film. The next stepin the continuous process was drying with forced air at a temperature of88° C. Thereafter, the filled porous film was stretched first in themachine direction and then in transverse direction, both at a stretchratio of 3:1. Machine direction stretching was at ambient temperatureand the transverse direction stretching at a range of 79 to 101° C.Characterization data is given in Table 5.

                  TABLE 5                                                         ______________________________________                                        Thickness (microns):                                                                             59                                                         Porosity (%):      84                                                         Tungsten content (%):                                                                            71                                                         ______________________________________                                    

Stacked layers of the biaxially oriented film were tested for X- rayabsorption efficiency using the 80 Kev bremstrahlung source. A 44-layerstack containing 94 mg/cm² showed a 99 percent efficiency.

In Comparative Example C1, an unfilled porous film made in the samemanner as in Example 4 except the dispersion volume was replaced with anequal volume of neat mineral oil. The thickness, density, and tensileproperties were determined. The results are set forth in Table 6.

                  TABLE 6                                                         ______________________________________                                                               Comparative                                                          Example 4                                                                              Example C1                                             ______________________________________                                        Thickness (microns):                                                                           59         42                                                Density (g/cc):    0.15       0.17                                            Tensile strength (psi):                                                       MD:             2270       1980                                               TD:             705        1270                                               Elongation (%):                                                               MD:             192        171                                                TD:             173        203                                                ______________________________________                                    

This data shows that a highly filled (71 percent tungsten) porous filmhas mechanical properties substantially equivalent to those of acomparable unfilled film.

COMPARATIVE EXAMPLE C2

An Ultra-Turrax Model SD-45 high speed shear mixer from Tekmar Co.,Cincinnati, Ohio, was used to mix mineral oil containing 5 weightpercent of 0.5 micron copper particles from Alfa Products, Danvers, Mass, for 1 hour at 100 percent power. Porous high density polyethylene filmwas prepared as in Example 1, with a melt blend ratio of 18.1:81.9polymer to dispersion. The cast film was washed with1,1,1-trichloroethane to remove the mineral oil and biaxially orientedas described in Example 1. Characterization data is given in Table 7.

                  TABLE 7                                                         ______________________________________                                        Thickness Microns):                                                                              150                                                        Porosity (%):      95                                                         Copper content (%):                                                                              16.5                                                       ______________________________________                                    

Backscatter SEM was used to characterize the film at both 500X and2000X. The results showed that most of the copper particles were inagglomerates of 5 to 10 microns in diameter and that the agglomeratedistribution was non-uniform.

This data can be compared to the backscatter SEM data of the filledporous film of Example 1 wherein the film was made in the same manner asthis example except the submicron particles were tungsten and weredispersively milled into the mineral oil. The 500X and 2000X SEM data ofExample 1 shows that most of the tungsten occurs as primary particlesabout 1 micron in diameter or smaller and that these are uniformlydispersed within the polymer. There were a few agglomerates of tungstencontaining 3 or 4 primary particles but these were no larger than 3microns in diameter. Photomicrographs of the SEM data at 2000X are shownin FIG. 2 for the copper of this example and in FIG. 3 for the tungstenof Example 1. The streaks in both photographs are artifacts produced byshadowing from the polyethylene matrix.

EXAMPLE 5

A mixture of 150 g tungsten powder (particle diameter 0.8 to 1.0microns, available from Teledyne Wah Chang, Huntsville, Ala.), 360 gdibutyl phthalate, and 5.0 g OLOA surfactant was prepared. Thedispersion was milled using the Igarashi attritor at 1800 rpm for 6hours. A blend of 394 g polyvinylidene fluoride (Soltex 1011, availablefrom Solvey, Inc.), and 394 g of the tungsten powder/dibutyl phthalatedispersion were batch mixed for 4 hours at 200° C. under a nitrogenatmosphere. The blend formed a homogeneous mixture which was pressedbetween plates heated to 200° C. to form a film approximately 0.6 mmthick which was quenched in a 5° C. water bath to initiate thermodynamicnon-equilibrium solid-liquid phase separation and solidification.

The cast film was washed with 1,1,1-trichloroethane to remove thedibutyl phthalate and then oriented 1.5×1.25 at 135° C. The resultingtungsten-filled microporous polyvinylidene fluoride film was evaluatedfor thickness, porosity, and tungsten content and the results are setforth in Table 8.

                  TABLE 8                                                         ______________________________________                                        Thickness (microns):                                                                             530                                                        Porosity (%)       47                                                         Tungsten content (%):                                                                            26                                                         ______________________________________                                    

EXAMPLE 6

A mixture of 150 g tungsten powder, (particle diameter 0.8 to 1.0microns, obtained from Teledyne Wah Chang, Huntsville, Ala.), 360 gtallow amine (Armostat 310, available from Armak Chemical Co., Chicago,Ill.), and 5.2 g OLOA surfactant was prepared. The dispersion was milledusing the Igarashi attritor at 1800 rpm for 6 hours. An additional 125 gof tallow amine was added to 402 g of the tungsten/tallow aminedispersion. A blend of 173 g polypropylene (Profax 6723, available fromHimont, Inc.), and 517 g of diluted dispersion were batch mixed for 4hours at 200° Cunder a nitrogen atmosphere. The blend formed ahomogeneous mixture which was pressed between plates heated to 180° C toform a film approximately 0.6 mm thick which was quenched in a 5° C.water bath to initiate thermodynamic non-equilibrium liquid-liquid phaseseparation and solidification.

The cast film was washed with 1,1,1-trichloroethane to remove the tallowamine and then oriented 1.5×1 at 135° C. The resulting tungsten filledmicroporous polypropylene was evaluated for thickness, porosity, andtungsten content and the results are set forth in Table 9.

                  TABLE 9                                                         ______________________________________                                        Thickness (microns):                                                                             580                                                        Porosity (%):      76                                                         Tungsten content (%):                                                                            38                                                         ______________________________________                                    

EXAMPLE 7

A mixture of 1350 g tungsten powder (particle size 0.8 to 1 micron,available from Teledyne Wah Chang, Huntsville, Ala.), 350 g mineral oilhaving a density of 0.87 g/cc, and 44 g OLOA 1200 surfactant wasprepared. A dispersion of the tungsten powder in the mineral oil wasachieved using the Igarashi attriter at 1800 rpm for 6 hours. 100 g ofthe dispersion was diluted with an additional 323 g of mineral oil. Ablend of 138 g polypropylene (Profax.sup.™0 6723, available from Himont,Inc.), 0.35 g nucleating agent (Millad™ 3905, available from MillikenChemical Co.) and 414 g of diluted dispersion were batch mixed for 4hours at 200° C. under a nitrogen atmosphere. The blend formed ahomogeneous mixture which was pressed between plates heated to 180° C.to form a film about 0.6 mm thick which was quenched in a 5° C. waterbath.

The cast film was washed with 1,1,1-trichloroethane followed byisopropyl alcohol to remove the mineral oil. The resultingtungsten-filled microporous polypropylene film was evaluated forthickness, porosity, and tungsten content. The results are set forth inTable 10.

                  TABLE 10                                                        ______________________________________                                        Thickness (microns):                                                                             600                                                        Porosity (%):      14.8                                                       Tungsten content (%):                                                                            42.6                                                       ______________________________________                                    

EXAMPLE 8

A mixture of 600 g tungsten powder (particle size 0.8 to 1 micron,available from Teledyne Wah Chang, Huntsville, Ala.), 300 g triethyleneglycol, and 10.5 g OLOA 1200 surfactant was prepared. A dispersion ofthe tungsten powder in the triethylene glycol was achieved using theIgarashi attriter at 1800 rpm for 6 hours. A blend of 170.5 g nylon 6(Nycoa™ 589, available from Nylon Corporation of America, Manchester,N.H.), and 500 g of dispersion were batch mixed for 4 hours at 220° C.under a nitrogen atmosphere. The blend formed a homogeneous mixturewhich was pressed between plates heated to 220° C. to form a film about1.2 mm thick which was quenched in a 5° C. water bath.

The cast film was washed with isopropyl alcohol to remove thetriethylene glycol. The resulting tungsten-filled microporous nylon 6film was evaluated for thickness, porosity, and tungsten content. Theresults are set forth in Table 11.

                  TABLE 11                                                        ______________________________________                                        Thickness (microns):                                                                             1200                                                       Porosity (%):      14.8                                                       Tungsten content (%):                                                                            58.2                                                       ______________________________________                                    

Various modifications and alterations of this invention will be apparentto those skilled in the art without departing from the scope and spiritof this invention and this invention should not be restricted to thatset forth herein for illustrative purposes.

What is claimed is:
 1. A method for preparing a particulate-filledmicroporous thermoplastic polymeric shaped article which comprises thesteps of(a) dispersing submicron or micron-sized particulate filler in aliquid compatible with the thermoplastic polymer to form a colloidalsuspension of the particulate filler in the liquid, the particulatefiller being substantially non-agglomerated; (b) melt-blending thethermoplastic polymer with a solubilizing amount of the compatibleliquid containing the dispersed particulate filler at a temperaturesufficient to form a homogeneous solution; (c) forming an article fromthe solution; (d) cooling the shaped article at a rate and to atemperature sufficient to initiate thermodynamic, non-equilibrium phaseseparation; (e) further cooling the article to solidify thethermoplastic polymer; and (f) removing at least a substantial portionof the compatible liquid, said particulate filler remainingsubstantially entirely within the thermoplastic polymer article.
 2. Themethod of claim 1 wherein said phase separation is liquid-liquid phaseseparation.
 3. The method of claim 1 wherein said phase separation isliquid-solid phase separation.
 4. The method of claim 1 furthercomprising orienting said article.
 5. A microporous particulate-filledthermoplastic polymeric article which comprises a thermoplasticpolymeric structure made by the method of claim 1 having a plurality ofinterconnected passageways to provide a network of communicating pores,the microporous structure containing discrete submicron or lowmicron-sized particulate filler, the particulate filler beingsubstantially non-agglomerated.
 6. The article of claim 5 wherein saidparticulate filler has a major axis no larger than 5 microns.
 7. Thearticle of claim 5 wherein said particulate filler has a major axis nolarger than 3 microns.
 8. The article of claim 5 wherein saidparticulate filler has a major axis no larger than 1 micron
 9. Thearticle of claim 5 wherein said particulate filler comprises up to about95 weight percent of said article.
 10. The article of claim 5 whereinsaid structure comprises void spaces encased by fibrous, lacy, orsemi-continuous boundaries.
 11. The article of claim 5 wherein saidstructure comprises a multiplicity of spaced, randomly disposed,non-uniform shaped, equiaxed particles of thermoplastic polymer,adjacent particles throughout said structure being separated from oneanother to provide the structure with a network of interconnectedmicropores and being connected to each other by a plurality of fibrilsconsisting of said thermoplastic polymer.
 12. The article of claim 5wherein said thermoplastic polymeric structure comprises an olefinicpolymer, a condensation polymer, or an oxidation polymer.
 13. Thearticle of claim 12 wherein said olefinic polymer is high densitypolyethylene, low density polyethylene, polypropylene,polyvinyl-containing polymer, butadiene-containing polymer, oracrylate-containing polymer.
 14. The article of claim 12 wherein saidcondensation polymer is polyester, polyamide, polycarbonate, orpolysulfone.
 15. The article of claim 12 wherein said oxidation polymeris polyphenylene oxide.
 16. The article of claim 5 wherein said articleis a film.
 17. The article of claim 16 wherein said film is oriented inat least one direction.
 18. The article of claim 5 wherein said articleis a fiber.
 19. The article of claim 18 wherein said fiber is oriented.20. The article of claim 5 wherein said particulate filler is a metal, ametal oxide or a carbonaceous material.
 21. The article of claim 20wherein said metal is lead, platinum, tungsten, gold, bismuth, copper orsilver.
 22. The article of claim 20 wherein said metal oxide is leadoxide, iron oxide, chrome oxide, alumina, titania or silica.
 23. Thearticle of claim 20 wherein said carbonaceous material is carbon black.24. The article of claim 5 wherein said structure has a uniform porositytherethrough.
 25. The article of claim 5 wherein said structure has agradient porosity therethrough.
 26. The article of claim 5 wherein saidparticulate filler is substantially uniformly distributed throughoutsaid structure.
 27. The article of claim 16 wherein said article furthercomprises at least one layer of material laminated to said film.
 28. Thearticle of claim 27 wherein said material is a woven, knitted ornonwoven fabric, a film or an additional layer of said structure. 29.The article of claim 5 further comprising an imbibed material.
 30. Thearticle of claim 29 wherein said imbibed material is a medicament, afragrance, an antistatic agent, a surfactant, or a pesticide.
 31. Amicroporous particulate-filled thermoplastic polymeric filtration mediacomprising a thermoplastic polymeric structure made by the method ofclaim 1 having a plurality of interconnected passageways to provide anetwork of communicating pores, the microporous structure containingdiscrete submicron or low micron-sized particulate filler, theparticulate filler being substantially non-agglomerated.
 32. Aprotective garment comprising a microporous particulate-filledthermoplastic polymeric fabric, the fabric comprising a thermoplasticpolymeric structure made by the method of claim 1 having a plurality ofinterconnected passageways to provide a network of communicating pores,the microporous structure containing discrete submicron or lowmicron-sized particulate filler, the particulate filler beingsubstantially non-agglomerated.
 33. X-ray shielding material comprisinga thermoplastic polymeric structure made by the method of claim 1 havinga plurality of interconnected passageways to provide a network ofcommunicating pores, the microporous structure containing discretesubmicron or low micron-sized heavy metal-containing X-ray absorbingparticulate filler, the particulate filler being substantiallynon-agglomerated.
 34. Electromagnetic shielding material comprising athermoplastic polymeric structure made by the method of claim 1 having aplurality of interconnected passageways to provide a network ofcommunicating pores, the microporous structure containing discretesubmicron or low micron-sized heavy metal electromagnetic absorbingparticulate filler, the particulate filler being substantiallynon-agglomerated.